jfoucher

This plugin uses the yahoo.com API to find the most relevant keywords from the content of your post, and then adds them as tags.
The tagging is fully automatic, so if you’re using a plugin like feedwordpress to display RSS feeds on your blog as posts, everything will get done as the feed posts are published. No user intervention necessary!

Auto-tags

Automatically add relevant tags to new posts.

freekrai

With the YQL Auto Tagger plugin, you’ll never have to think of tags for your posts again. The plugin uses the Yahoo Query Language to perform semantic analysis of your post text and suggest tags for you. Add them to your post with just a click.
This plugin requires PHP 5 and the cURL library (both of which are available on most web hosts).
Based on the Open Calais Auto Tagger by Dan Grossman – http://www.dangrossman.info

YQL Auto Tagger

The plugin performs an analysis of your post text and suggests tags for you.

Auto-tags vs YQL Auto Tagger